Diffusion pump and mass spectrometer



M. E. REINECKE ETAL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 1 March 31, 1959Filed June 3. 1955 MOLECULAR LEAK STANDARD SUPPLY VACUUM PUMP 3o l3 Q SL 'L. ./3a 37 BOOSTER VACUUM PUMP PUMP II INVENTORS. I M. E. REINECKE A.B. BROERMAN WJM March 31, 1959 M. E. REINECKE ET AL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER Filed June 5, 1955 5 Sheets-Sheet 2f4 1 F G. 3

Y 65 64 72 7o 66 y 5 46 69 INVENTORS.

M. E. REINECKE A. a. BROERMAN A7 RNEY March 31, 1959 M. E. REINECKE ETAL 2,

DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 3 Filed June 5, 1955INVENTORS. M. E. REINECKE A. B. BROERMAN H JM Mar h 31, 1959 M. E.REINECKE ET AL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER Fil ed June 3, 1955 5 Sheets-Sheet4 PLANT STREAM INPUT I IOO CLEANING SYSTEM VENT FLOWRATOR a4 jlOl .9-F92 [[9 PC 23 I4 i VISCOUS j LEAK *1 5 l5 STANDARD FLUID MOLECULAR eSUPPLY LEAK SAMPLE swn'c L I04 r VACUUM I06 V I05 ION GAUGE DIFFUSION-:\39 4 PUMP i1 '3' WATER no VACUUM BOOSTER PUMP PUMP I09 I32J I30 me asHla v 123a FIG. 6 113 INVENTORS.

M. E. REINECK'E A. B. BROERMAN BY ATTO NEYS 'March 31, 1959 M, E.RElNECKE ET AL DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 5Filed June 3, 1955 CIRCUITS l4 J (ELECTRONIC HEATER\ FIG. 8

IN VEN TORS. M. E. REINECKE A. B. BROERMAN ATTOR EYS.

FIG. 7'

United States Patent DIFFUSION PUMP AND TMASS SPECTROMETER MarvinE..Reinecke and Arthnr.B..Broerman, Bartlesville,

0kla., assignors to Phillips Petroleum Company, a corporation ofDelaware Application June 3, 1955, iSeriaLNo. 513,008 10 Claims. '(Cl.250-419.)

This invention relates to mass spectrometers particularly suited forindustrial applications. .In another aspect, it relates to adiftusionpump.

Heretofore, mass spectrometers have been used extensively as laboratoryequipment for conducting various types of analyses. .Recently, interesthas developed in the industrial application of mass spectrometers asfactory .or refinery analytical and/or control instruments. However,where flammable or explosive vapors are present in the atmosphereadjacent the :unit, thereis a distinct explosion hazard. v

:In accordance with this invention, a commercially practical massspectrometer unit is provided which is suitable for use in hazardouslocationaparticularly where flammable or explosive vapors may bepresent. The mass spectrometer tube .is enclosed wit-hinanexplosion-resistant metal housing, and a vacuum conduit is connected tothe tube interiorly of the housing and extends outwardly to a valve anddifi-usion pump connection whereby the requisite degree of vacuum can bemaintained .in the spectrometer tube. Advantageously, this vacuumconduit extends through a special cylindrical flanged section forming apart of the explosion-resistant housing. The vacuum conduit and valveare, in turn, connected to a 'difiusi'on pump having an exteriorelectrical heating unit which is enclosed within a specialexplosion-resistant housing from which the electrical leads arewithdrawn through a conduit. The mechanical pump, which acts as a forepump for the diffusion pump, is driven by an explosion-proof electricalmotor, as is a pump provided in the sampling system, a part of which ismounted outside the casing. The solenoid valve and other controlelements of this sampling system are likewise of explosion-proofconstruction.

I Accordingly, it is an object of the invention to provide a massspectrometer unit capable of being used in haz ardous locations whereflammable or explosive vapors may be present.

It is a further object to provide a diffusion pump which can be safelyused in such locations.

It is a still further object to provide such units in an economical way,and yet provide the requisite degree of protection.

Various other objects, advantages and features of the invention willbecome apparent from the following detailed description taken inconjunction with the accompanying drawings, in which:

Figure 1 is a schematic diagram of the mass spectrometer system; H

Figure 2 is a vertical sectional view, partially in eleva tion, of themass spectrometer, difiusion pump and associated elements;

- Figure 3 is a vertical 'view, partially in elevation, of the bottomportion of the diffusion pump;

Figure 4 is a front elevational view, partially in section, of amodified mass spectrometer system;

. Figure 5 is a side elevational view of the modification of Figure 4;

2,880,323 Patented Mar. 31, 1959 Figure 6 is a schematic flow diagram ofthe mass spectrometer system of Figures 4 and 5; I

Figure 7 is a vertical sectional view of a modified -diffusion pumpheater assembly; and

Figure 8 is a schematic circuit diagram illustrating a feature of theinvention.

Referring now to Figure 1, a mass spectrometer tube 10 is enclosedwithin an explosion-resistant metal housing 11 hereafter described indetail, this tube being connected to a sampling system generallyindicated by reference numeral 12 and an exhaust system generallyindic'ated by reference numeral 13. This tube can advant'a'geously be ofthe type shown in copending application of M. C. Burk entitled IonSource, Serial Number 412,790, filed February 26, 1954, now U.S. Patent2,792,500, and the associated electronic circuits described in thatcopending application are mounted interiorly of the housing 11.

. The sampling system 12 includes an inlet line 13a which is connectedthrough a viscous leak '14 and a molecular leak 15 to the interior ofthe tube 10. The inlet line 13a is further connected by a solenoid valve16 of explosion-proof construction to a sample line 17 which isincorporated a flow controller 18 and a valve 19. The solenoid valve 16is also arranged to connect the sample line 17 to a pipe 20 which leadsto a vent conduit 21 whereby sample vapors bypassing the massspectrometer during the standardization cycle hereafter referred to aresafely vented.

The inlet line 13a is further connected by an explosion proof solenoidvalve 22 to a standard fluid line 23 which incorporates a fiowcontroller 24 and is connected to a standard fluid source 25. w

The inlet line, adjacent the molecular leak 15, is fun ther connected toan exhaust line 27 which leads through a special tubing section 28 ofproper diameter and length to regulate the pressure at the molecularleak 15 to a desired value. It was found that provision of a needlevalve instead of the tubing section 28 caused undesirable molecular flowin the viscous flow line. The special tubing section 28 is, in turn,connected to a rotary vacuum pump 30 driven by an explosion-proofelectrical motor 31. Just upstream of the viscous leak 14, the inletline 13a is bypassed to the vent conduit 21 through a flow controller32.

In the operation of the sample system, valves 16 and 22 are operatedalternately. During an indicating cycle, sample flows through lines 17and 13a to the interior of the tube where it is ionized by electronsemitted from an incandescent filament, and ions of a particular mass areselectively accelerated toward a collector electrode which is connectedto an indicating circuit showing the concentration of ions of a selectedmass in the sample stream. Either alternatively or in conjunction withthe indicating function, the output can be applied to control apparatusto control one or more process variables to maintain a predeterminedconcentration of ions of a selected mass number in the sample stream.During the standardizing cycle, solenoid valve 16 is actuated to divertthe flow of sample fluid through pipe 20 to the vent conduit 21 and tocause standard fluid to flow through line 23, valve 22, and line 13a tothe interior of the tube. During this standardizing cycle, the gain ofan amplifier associated with the tube is varied to compensate for anydrift which might have occurred during the preceding indicating cycle.

Both the sample fluid and standard fluid llo'w through the viscous leak14 and molecular leak 15. The flow 0 through leak 14 is viscous, thatis, the path length through the leak is long compared with the meanfree. path of the molecules in the stream so that flow through.

3 the leak depends upon the viscosity of the fluid, the pressure, andthe dimensions of the opening.

At the molecular leak 15,-the opening is of molecular dimensions, andflow therethrough depends upon the geometry of the system and themolecular weight of the molecules rather than upon viscosity of thefluid. A fractionation effect occurs in the molecular leak, molecules oflower molecular weight being preferentially transmitted through theopening defined thereby. However, since the flow out of the tube ismolecular flow, as distinguished from viscous flow, this fractionationefiect does not vary the relative proportions of components of differentmolecular weight entering and leaving the tube.

Both the viscous leak and molecular leak are required because it isimpractical to go from a region of atmospheric pressure through amolecular leak directly to the interior of the tube. Thus, the pressureis substantially reduced by the viscous leak, so that there is viscousflow downstream thereof through the exhaust line 27, which viscous flowis not affected by the small amount of material passing to the interiorof the tube through the molecular leak 15. This combination of molecularand viscous leak provides a practical and efficient way of introducing arepresentative sample of the stream passing through inlet line 17 orstandard fluid line 23 to the interior of the tube.

Material leaves the tube through a conduit 35, flow in which is governedby the molecular weight of the molecules and geometry of the system,rather than the viscosity. This material passes through the exhaustsystem 13 which includes a globe valve 36 with a bellows sealed stem, abattle assembly 39 and a diffusion pump 38. The high pressure side ofthe diffusion pump is connected by a conduit 37 to a booster pump 40 anda rotary vacuum pump 41, the latter pump being preferably driven by themotor 31 which is explosion-proof as previously noted. These unitsremove the gases being pumped by the diffusion pump.

The diffusion pump has a plurality, for example three, of jets throughwhich a diffusion pump oil is ejected at high velocity, the oil beingvaporized by an external electrical heater, the baffle system 39preventing this diffusion pump oil from entering the tube 10 with theexception of a small amount which gets into the system by evaporationfrom the surfaces of the baffle. The difluslon pump can be provided witha pump oil fractionation system, and other auxiliary components as thoseskilled in the art will readily understand, and the combination of pumpsprovides a high order of vacuum interiorly of the tube 10.

Preferably and advantageously, the tube 10 has a metal envelope so thatno breakage will occur with resultant discharge of flammable orexplosive vapors into the interior of the housing 11. However, importantadvantages of the invention are realized where the tube has a glassenvelope.

The construction of the explosion-proof housing and the tube andassociated electrical circuitry is shown in detail by Figure 2. It willbe noted that a metal frame 45 having a base 46 carries an end plate 47having an opening 48 therein to which the tube 10 can be inserted orwithdrawn, this opening being closed and sealed by a cap 49. Secured tothe plate 47 is a flanged cylindrical member 50 which, in turn, issecured by bolts to a generally cylindrical flanged housing section 51having a cylindrical body portion 52 with a dome-shaped integral endsection 53. The members 47, 50 and 51 defining the housing are formedfrom explosion-resistant metal, such as steel, and the joints betweenthe parts are substantially vapor tight. This is in accordance withspecifications which provide for breathing" at the joints but do notallow flame propagation therethrough.

The'tube 10 is secured by a mounting fixture 54 to a support 55 which ispositioned below a shelf 56 carrying electronic equipment 57 associatedwith the tube. The

tube envelope defines a cylindrical outlet section 58 which,

in turn, is connected and sealed to a flanged pipe 59 extending radiallythrough the cylindrical section 50 and welded thereto to form a vaportight seal. The protruding end of the pipe 59 is connected through avacuum globe valve 60 and a horizontal conduit, not shown, to the lowpressure side of the diffusion pump 38, the high pressure side of whichis connected to a conduit 61 leading to the booster pump 40 and rotaryvacuum pump 41 of Figure 1.

The body of the difiusion pump 38 is defined by a generally cylindricalcasing 62 having a lower end plate 63 to which an annular electricalheater element 64 is secured by a nut 65 and a bolt 66.

The heater 64, in turn, is enclosed by an annular housing 67 having itsupper end welded at 68 to the lower portion of the casing 62. An endplate or disc 69 is secured to the bottom of the housing member 67 sothat a joint which permits breathing but prevents flame propagation isformed.

The terminals of the heating element, one of which is indicated at 70,are connected to leads 71, 72 which extend through a metal pipe orconduit 73. The latter conduit extends through the side wall of thehousing section 67 and forms a vapor tight seal therewith.

It will be evident that we have achieved the objects of our invention inproviding a complete mass spectrometer unit suitable for use inhazardous locations where flammable or explosive vapors may be present.In particular, although the enclosure defined by the elements 47, 50 and52 is essentially vapor tight, some flammable or explosive vapors mayleak into the interior of this enclosure. Should these vapors be ignitedby a spark from the electronic equipment in the enclosure, the wallsthereof effectively confine the explosion so that it is not transmittedto the area outside the equipment. Of course, this will only occurrarely, and it has been found that the enclosure effectively performsthe desired function of confining or minimizing any flame or explosionwhich may occur so that it does not affect the plant exteriorly of theinstrument.

In fact, in testing equipment of the present type, explosive mixtureswere purposely introduced to the interior of the enclosure and anexplosion was produced therein.

This explosion was not sufliciently severe that it would damagesensitive tube and electronic circuit elements in the enclosure.Moreover, no spark or flame was transmitted to the region outside theenclosure.

The same remarks apply to the enclosure of the electrical heater element64 on the difiusion pump. Were this not provided, a spark occurring inthe heater leads might ignite vapors in the atmosphere surrounding theinstrument and cause a dangerous fire or explosion, which is effectivelyprevented by provision of the metal housing 67, 69 and its associatedconduit 73 carrying the electrical leads to the heater element.

Finally, it should be pointed out that even if the envelope of tube 10breaks, assuming it is formed from glass or other breakable material,and flammable vapors from the sample line are discharged into theinterior of the housing, igniting of the vapors and any resulting ex-'plosion will still be effectively confined within the region defined bythe members 47, 50 and 52.

Referring now to Figures 4 to 8 inclusive and par ticularly to Figure 6,we have shown a modified mass spectrometer system which is similar insome respects to that described by Figures 1 to 4 inclusive and in whichthe corresponding parts are indicated by like reference numerals.

In this system, the sample before being admitted to the line 17 passesthrough a sample cleaning system 100. Here, a solenoid valve 101 isprovided to connect either line 17 or 23 to the line 13 and a secondsolenoid valve 102 connects line 17 to the vent 21, the same functionbut being the valves 16, 22 of Figure 1.

these valves having arranged diflercntly than In this modification, thetube has an explosionproof cover or housing 103, and the interior of thetube communicates through aline 104 and valve 105 with the maindiffusion pump 38 vand its associated batfie 39.

An 1011 gage 106 surrounded by an explosion-proof cover 107 communicateswith the conduit 104, and a vacuum switch 108 also communicates with theinterior of conduit 104 and thus with the interior of the tube 10. T1118vacuum switch is arranged to be actuated when the pressure in the tuberises above a predetermined va ue. The main diifusion pump 38issurrounded by a water 1&Ck6t 109 having a thermal switch 110 inthermal contact therewith, this switch being actuated when the waterjacket temperature rises above a predetermined value.

The heater assembly at the bottom of the main diffusion pump, indicatedgenerally by reference numeral 11, is of modified construction, as shownby Figure 7. Referring to this figure, it will be noted that housing 112is suitably secured, as by welding, tothe lower end of the cylindricalhousing 62 of the main diffusion pump 38.

An annular heater 113 is secured to the bottom of the casing 62 by a nut114 threaded to a member 115 which defines an oil drain 117 at thebottom of the unit, a sealing ring 116 being provided at the lower endof the member 115 and being held in place by a bolt 117. As in thestructure of Figure 3 the heater leads 119 extend through an opening 120in the side of the housing 112 which can receive a suitable metalconduit.

Insulating strips 121, 122 are mounted interiorly of the housing 112,and a thermal switch 123 is mounted in a recess 124 defined by theinsulating strip 122 and a plate 125 secured to the lower end of thehousing, the leads 126 of the thermal switch extending through theopening 120.

The thermal switch 123 is actuated when the housing temperature risesabove a predetermined value.

The high pressure side of the diffusion pump is con nected by a conduit128, Figure 6, and a flame arrestor 129, which can be of the capillarytube type or of any other suitable type, to a booster diffusion pump 130having a housing assembly 111a at its lower end of the same constructionas described in connection with Figure 7, this unit including a heater113a and a thermal switch 123a. The high pressure side of the boosterdilfusion pump 130 is connected by a conduit 131 to a rotary vacuum pump132.

Referring now to Figure 8, it will be noted that the switches 108, 110,123 and 123a each have one grounded terminal, and have their otherterminals connected, respectively, to the operating windings of a seriesof relays 133a to 133d, each operating winding further being connectedto ground through a suitable current source repre sented by therespective batteries 134a to 134d.

Each relay has a holding circuit, and these holding circuits are definedby the respective normally open contact sets 135a to 135d and resetswitches 136a to 136d.

The relays further have the respective sets 137a to 137d of normallyclosed contacts, all of which are connected in series with a power plug138 and a fuse 139 so that a current is supplied to the heaters 113,113a and to the amplifier and associated electronic circuits 140 of themass spectrometer unit when all of the contacts 137 are closed. However,opening of any one of these contact sets interrupts the supply ofcurrent to the heater and electronic circuits. Moreover, when any of therelay 133 is actuated, its associated holding circuit maintains therelay in energized condition until it is reset by actuation of theassociated reset switch 136.

In operation, it will be noted that switch 108 closes whenever apredetermined degree of vacuum does not exist within the tube 10, Figure6. Thus, should vacuum be lost during operation, or should the filamentinadvertently be turned on before evacuation of the tube when flammableor explosive vapors might be present, switch 108 is closed and theheater and electronic circuitsare broken 'so that no spark can be .setoff with resultant danger ,of an explosion.

Moreover, if the water supply to the jacket I109 fails, switch isactuated to interrupt the supply of current to the heater and electroniccircuits. Finally, should the temperature within the housing 111 or'lllarise above ,a predetermined value, say 450 B, one of the switches123 or 123a is energized to interrupt the flow of current to the heaterand electronic circuits. Accordingly, we have provided an interlockingsafety system to prevent or greatly minimize explosion hazards shouldthe vacuum in the tube fail, the diifusion pump temperatures 'rise to adangerous value, or the flow of water throughthe diffusion pump coolingjacket be interrupted.

Referring now to Figures 4 and 5, it will be noted that the cylindricalhousing 53 encloses the electronic circuits and amplifier in anexplosion-resistant housing. The tube 10, instead of being mounted inthe housingJ53 {is supported by a shelf within an explosion-resistanthousing 151. If desired and preferably, electronic circuits making up apreamplifier can beincorporated within the upper part 152 .of thehousing.

Within the housing and surrounding the tube is a sheath 153 ofinsulating material, and the tube is mounted vertically so that the tubeoutlet is horizontal, rather than vertical as in the system of Figure 2.The entire apparatus is supported by a base 154 upon which are mountedthe booster diffusion pump 130, the main diffusion pump 38, the flamearrestor '129 and the rotary vacuum pump 132. L

In this structure, the housing 151 is of relatively small volume, forexample, about 200 cubic inches.

Where the tube is formed from glass, it is evident that it may burst ifan explosion would occur within the housing 151. To minimize thepressure buildup due to an explosion, this housing is made to fitclosely around the tube because a small volume will not produce as highan explosion pressure as a large enclosure. Moreover, if the tubebursts, the relatively large volume of the diffusion pump will allowexpansion of the gasesinto the evacuated pump chamber. Thus, thepressure buildup inside the diffusion pump 38 will be small (not morethan about 90 pounds per square inch). The diffusion pump is ofexplosion-proof construction to prevent flame from escaping and towithstand pressure surges. The flame arrestor 129 prevents flame frompassing beyond the dilfusion pump and also clamps pressure surges sothat the pressure buildup in the booster and rotary pumps is notexcessive.

Alternatively, a venting section or flame arrestor is placed in theexplosion-proof cover around the tube to allow blowdown to theatmosphere without allowing flames to escape.

While the invention has beendescribed in connection with present,preferred embodiments thereof, 'it is to be understood that thisdescription is illustrative only and is not intended to limit theinvention.

We claim:

1. In a mass spectrometer, in combination, an explo sion-resistanthousing, a mass spectrometer within said housing including a tube, amain diffusion pump connected to said-tube through said housing, abooster diffusion pump connected to said main diffusion pump', a heaterconnected to the housing of each diffusion pump, a housing enclosingeach such heater, a thermal switch in each housing which is actuatablewhen the housing: temperature rises abovei-a predetermined value,meansfor supplying an electrical current to said heater, and means forinterrupting said supply of current upon actuation of either switch.

2. In a mass spectrometer, in combination, an explosion-resistanthousing, a mass spectrometer within said housing including a tube, amain difiusion pump connected to said tube through said housing, abooster dif fusion pump connected to said main diffusion pump, a heaterconnected to the housing of each diflusion pump, a housing enclosingeach such heater, a thermal switch in each housing which is actuatablewhen the housing temperature rises above a predetermined value, meansfor supplying an electrical current to said heater, a water jacketsurrounding the housing of said main diffusion pump, a thermal switch inthermal contact with said water jacket and actuatable when thetemperature therein rises above a predetermined value, means forsupplying an electrical current to each of said heaters, and means forinterrupting the flow of current through said heaters upon opening ofany one of said thermal switches.

3. In a mass spectrometer, in combination, an explosion-resistanthousing, a mass spectrometer within said housing including a tube, amain diffusion pump connected to said tube through said housing, abooster diffusion pump connected to said main diffusion pump, a heaterconnected to the housing of each ditfusion pump, a housing enclosingeach such heater, a thermal switch in each housing which is actuatablewhen the housing temperature rises above a predetermined value, meansfor supplying an electrical current to said heater, a water jacketsurrounding the housing of said main diffusion pump, a thermal switch inthermal contact with said water jacket and actuatable when thetemperature therein rises above a predetermined value, means forsupplying an electrical current to each of said heaters, means includinga relay connected to each thermal switch to interrupt the flow ofcurrent to said heaters upon actuation of any thermal switch, and aholding circuit connected to each relay.

4. In a mass spectrometer, in combination, an explosion-resistanthousing, a mass spectrometer within said housing including a tube, adiffusion pump connected to said tube through said housing, a heaterconnected totthe housing of said difiusion pump, a housing enclosingsaid heater, a thermal switch in said housing which is actuatable whenthe housing temperature rises above a predetermined value, means forsupplying an electrical current to said heater, a vacuum switchcommunicating with the interior of said tube and actuatable when thetube pressure rises above a predetermined value, and means responsive tothe actuation of said vacuum switch to interrupt the supply of currentto said heater.

5. In a mass spectrometer, in combination, a general cylindricalexplosion-resistant housing having a flanged open end, a flangedcylinder formed of explosion-resistant material secured to the open endof said housing, an end plate sealing the open end of said cylinder, amass spectrometer within said housing including a tube, a vacuum conduitextending radially through said cylinder and connected to said tubeinteriorly of said cylinder, means sealing the area between saidcylinder and said vacuum conduit, said conduit having a flanged endprotruding from said cylinder, a flanged vacuum valve secured to saidflanged end, a diffusion pump connected to said vacuum valve, saiddiffusion pump including a casing, an exterior housing enclosing thelower end of said casing and formed from explosion-resistant metal, acylindrical heater disposed in contact with the lower end of said casingand positioned within said housing, a conduit communicating with saidhousing, leads extending through said conduit into said housing andconnected to said heater, and means sealing the area between saidhousing and said casing.

6. The apparatus of claim 5 wherein the mass spectrometer tube has anexplosion-resistant metal envelope.

7. A mass spectrometer including, in combination, an explosion-resistantmetal housing, a mass spectrometer tube within said housing, a vacuumconduit extending through said housing and connected to said tube, adiffusion pump mounted exteriorly of said housing and connected to saidvacuum conduit, a mechanical pump connected to said ditfusion pump, asample inlet line, a flow controller in said line, a source of standardmaterial, a standard fluid line connected to said source, a flowcontroller in said standard fluid line, an inlet line having a viscousleak therein and a molecular leak communicating with said tube, saidinlet line extending through said housing, a vacuum pump connected tosaid inlet line downstream of said molecular leak, a vent conduit,solenoid valve means connecting said sample line selectively to saidvent conduit and to said inlet line upstream of said viscous leak, andselectively connecting said standard fluid line to said inlet lineupstream of said viscous leak, said solenoid valves being of theexplosion-proof type, and an explosion-proof electric motor driving saidvacuum pumps.

8. An explosion-proof difl usion pump including, in combination, acasing having a flat-bottomed section, diffusion pump mcchanism in saidcasing, an explosion-resistant housing secured to said casing andenclosing the fiat-bottomed section thereof, a cylindrical electricalheater secured to said flat-bottomed section exteriorly thereof andpositioned within said housing, a conduit extending through saidhousing, and leads extending through said conduit and connected to theterminals of said heater. I

9. An explosion-proof diffusion pump which comprises, in combination, anupstanding cup-shaped casing, diffusion pump mechanism in said casing, agenerally cylin-.

drical housing having a cylindrical section welded to the sides of saidcasing adjacent the lower end thereof, said housing enclosing the lowerend of said casing, a plate sealing the lower end of said housing, anannular electrical heater positioned against the outside of said cas-.

ing, a bolt securing said heater to said casing, a conduit extendingthrough the side walls of said housing, and leads extending through saidconduit into the housing and connected to the terminals of said heater.

10. An explosion-proof diffusion pump including, in combination, acasing having a flat-bottomed section, ditfusion pump mechanism in saidcasing, an explosionresistant housing secured to said casing andenclosing the flat-bottomed section thereof, a cylindrical electricalheater secured to said flat-bottomed section exteriorly thereof andpositioned within said housing, a conduit extending through saidhousing, leads extending through said conduit and connected to theterminals of said heater, and a thermal switch positioned in saidhousing, and actuatable when the temperature therein rises above apredetermined value.

References Cited in the file of this patent UNITED STATES PATENTS2,024,726 Ehrenfeld Dec. 17, 1935 2,318,786 Korte et al. May 11, 19432,368,492 Ralston Jan. 30, 1945 2,702,869 Felici Feb. 22, 1955 2,768,584Nicol et al. Oct. 30, 1956

